1. Field of the Invention
This invention relates to a process and a method for treating and separating methane and carbon dioxide gases from landfill gases gathered at a landfill site.
As it is told, landfill gases when released into the atmosphere become a source of global warning greenhouse gas and smog-forming volatile organic gaseous emissions. The present Environmental regulations require that Landfill sites must be equipped with approved gas collection systems to control and prevent release of odors and landfill gaseous products into the environment. A gas collection system is employed to provide a negative pressure to pull out the landfill gas and to maintain low surface concentrations of gas at the ground surface, the collected gas is normally burned in boilers or flared into the atmosphere,
A typical landfill collection gas system consists of vertical and horizontal wells collecting gas from decaying organic matter at various levels undergound, the wells are connected by a pipe header at the ground surface. Oxygen sensor continuously monitor potential air migration and controls the landfill collection process to prevent atmospheric air from entering the system.
A typical landfill gas contains by volume basis an average of approximately 55% methane, 40% carbon dioxide, 2.3% nitrogen, 0.6% oxygen, 2% water vapor, less than 100 parts per million (PPM) of hydrogen sulfide and other insignificant smaller amounts of sulfur and hydrocarbon compounds.
In the present invention landfill gas is processed for treating and separating the methane and carbon dioxide to produce high quality liquified natural gas (LNG), liquefied carbon dioxide and compressed natural gas (CNG) products. A higher Octane more uniform methane fuel (natural gas) may be produced and conveyed into the natural gas utility pipe lines for domestic use, maybe produced as Compressed Natural Gas (CNG) for fueling vehicles similar to motor cars, tracks, busses, etc. or maybe produced as Liquified Natural Gas (LNG) to drive heavy equipment similar to railroad locomotives and marine entries, and for other uses that provide both economic and environmental benefits.
Carbon Dioxide gas maybe processed to produce liquefied carbon dioxide gas products that maybe tracked off site or conveyed by a pipe line to remote chemical manufacturing facilities for further processing and manufacturing of chemical products. The carbon dioxide separated from the landfill gas will replace a part of industrially produced carbon dioxide that require burning fossil fuel, thus providing the potential for both economic and environmental benefits.
A typical landfill site may produce between 1.0 and 15.0 million standard cubic foot per day (MMSCFD) of land fill gas. A system that treats 1.0 MMSCFD of landfill gas may produce up to 5,000 gallons per day of Liquefied Natural Gas (LNG) and 20 tons per day of Liquid Carbon Dioxide.
In the present invention, gas compression and cooling processes are employed to liquefy and separate most of the carbon dioxide gas from the methane gas. The landfill gas received from a gas collection system is cleaned by activated carbon to remove most of the hydrocarbon gas contaminants contained therein. The gas flowing from the low pressure cleaning step is then received by a multi stage gas compressor, where the gas pressure is increased and simultaneously cooled in four stages of compression and aftercoolers. The number of compression stages and the range of the cooling temperatures to liquefy the carbon dioxide gas contained in the landfill gas mixture will vary depending upon the chemical composition of landfill gases and the required degree of the methane gas purification through the compression and cooling steps. The carbon dioxide contained in the mixture of the landfill gas maybe condensed and removed at system pressures above 1800 pisg and at condensing temperatures that vary depending upon the content of the carbon dioxide in the landfill gas mixture. A liquid chiller or cryogenic refrigeration system is used to cool the gas to below 20.degree. F. temperature to achieve more than 85% carbon dioxide removal. The more volatile gas impurities similar to methane and hydrocarbons contained in the liquified carbon dioxide flowing from the condensing step is then vented by expanding the low temperature liquid carbon dioxide from 1800 psig pressure level to approximately 200 psig without forming ice. Physical adsorption of trace impurities in activated carbon bed maybe used to produce high quality, product. A methane rich gas mixture containing more than 90% methane and less than 10% carbon dioxide flowing from the fourth stage compression is then conducted to a carbon dioxide absorption system, where an absorption fluid (solvent or amine based fluid) removes more than 95% of the remaining carbon dioxide from the methane gas. A typical gas stream treated for producing Liquid Natural Gas (LNG) must be purified to a minimum of 95% methane and with not more than 0.5% carbon dioxide. Liquid carbon dioxide separated after the fourth stage compression aftercooler maybe further purified by adsorption to provide a high grade quality product. Carbon dioxide and methane gas mixture released from regenerating the absorbing fluid in multiple step flashing vessels and from a low pressure regenerator vessel is recycled back to combine with the landfill main gas stream entering the multi-stage compression for 100% recycling and removing the carbon dioxide gas.
Liquefied Natural Gas is produced by cooling and condensing the methane gas to below -200.degree. F. by exchanging heat with a cryogenic gas circulating in a closed turbo expander-compressor system. The cryogenic cooling effect must be enough to liquefy more than 80% of the compressed methane gas. The uncondensed methane contaminated with oxygen and nitrogen gases are vented and utilized for fueling the gas driven engines for the gas compression steps. The cryogenic gas stream flowing from a heat exchange system is conducted in a closed circuit to a gas compressor driven by the gas expander to provide dynamic loading to the expander. Compressed natural gas (CNG) at high pressure of up to 5000 psig for fueling automotive vehicles is produced by pumping a stream of the liquid natural gas (LNG) to the required high pressure level and by evaporation of said high pressure liquid in a heat exchanger using an auxiliary heat source.
2. Description of the Prior Art
The present invention employs a combination of gas compression and cooling processes to achieve separation of gas components at their thermodynamic equilibria points. The invention also employs gas purification and absorption process to remove trace amounts of carbon dioxide that is not economically possible to be removed by the liquefaction process and remains in the gas mixture steam flowing from the fourth stage compression and cooling step.
In the prior art, methods of removing carbon dioxide from landfill gas are limited to chemical or physical absorption and preamble membrane separation which occurs at much lower operating gas pressures. A chemical or physical absorption precesses typically employ an aqueous alkanolamine solution or a solvent to contact the gas stream in a trayed or packed vessel (the fluid contactor). The amine solution is a weak organic base which removes the carbon dioxide from the gas stream. The CO.sub.2 -rich amine stream which is loaded with carbon dioxide is heated and flashed at much lower pressure into a second trayed vessel (the regenerator) to produce a CO.sub.2 -lean amine. The combination of lower pressure and higher temperature cause a reversal of the chemical reactions which occurred within the fluid contactor, carbon dioxide is released from the amine solution or the solvent fluid and is vented through the top of the regenerator. Advantages of the chemical or physical absorption processes are achieving low concentrations of carbon dioxide in the methane gas; disadvantages, include high capital and operating costs, high fuel consumption, complexity of operations and costly oversized equipment to remove high content of carbon dioxide (30% or more by volume).
In permeable membrane process, membranes separate gases by selective permeation of the gases in contact with the membrane. The gases move across the membrane barrier as a result of imposed partial pressure gradients. The gases are separated based on diffusivity through the membrane material. The membrane material can be one of several molecular sieves depending on the composition of the mixture of gases to be separated. Higher quality and purity of product, require two or more stages of membrane separators and recycling intermediate concentrations of gas stream back to the inlet of the first stage membrane system. Advantage for using permeable membranes are ease of operation and a higher degree of gas separation is achieved. Disadvantages include higher initial cost, higher maintenance cost, higher operating cost, expensive replacements of membranes, and costly oversized equipment to recycle and reheat a large percentage of the gas stream entering the first membrane stage.
Neither, the carbon dioxide gas absorption nor the permeable separation processes for treating a landfill gas containing 30% (vol.) or more of carbon dioxide, has proven to be economically attractive for treating landfill gases from sites that produce less than 5 MMSCFD specially when additional costs will be needed to compress the treated methane gas for producing liquid natural gas (LNG) and compressed natural gas (CNG). The present invention utilizes the energy needed for the methane gas and carbon dioxide gas liquefaction process and takes advantage of most of this energy to substantially separate the bulk of the carbon dioxide contained in the compressed landfill gas stream, thus reducing the energy needed for the absorption step by reducing the percentage of carbon dioxide to be absorbed.
At higher pressures and at lower temperatures carbon dioxide gas progressively condenses as its thermodynamic points of equilibria that correspond to its content in the mixture of the compressed landfill mixture of gases. The process takes advantage of the energy needed to produce compressed natural gas (CNG) and liquid natural gas (LNG) to simultaneously remove the carbon dioxide as liquefied carbon dioxide (liquid CO.sub.2) product, providing an economical solution for the landfill gas treatment and utilization.
The present invention employs a thermally regenerative organic based amine absorbent or solvent fluid to absorb most of the trace amount of carbon dioxide contained in the methane rich gas stream flowing from the carbon dioxide liquefaction step. Regenerative absorbents and solvents have been used in the past for scrubbing carbon dioxide (CO.sub.2) hydrogen sulfide (HSO.sub.2) and other landfill gas contaminants. Well known thermally organic amines as Monoethanlamine and Diethanolamine have been widely used for CO.sub.2 absorption. Commercially known carbon dioxide absorbent products similar to Selexol.TM. has been developed by Union Carbide Corporation.
In the present invention a methane rich gas mixture flowing at above 1800 psig containing less than 10% carbon dioxide is treated with an organic amine absorbent (similar to Selexol.TM.) to remove the carbon dioxide at a relatively low liquid to gas ratio (L/G), resulting in a lower energy consumption rate, and reduced power requirements. The process in the present invention for treating landfill gas provides the combination of producing compressed national gas (CNG) while simultaneously removing the carbon dioxide; firstly by compression and cooling, and secondly by absorption of the remaining low content of carbon dioxide in the mixture of gas stream flowing from the carbon dioxide liquefaction step.
A preferred embodiment of the present invention is the absorption step of the CO.sub.2 which occurs at a relatively higher pressure above 1800 psig and the multiple flash regeneration of the absorbing fluid that occurs in multiple flashing vessels vented to the suction side of corresponding suction pressure of multiple compression stages to reduce the pressure of the absorbing fluid and recycle the vented gas. The high partial pressure of the carbon dioxide in the absorber, and in the regenerator improves the absorption and regeneration rates and reduces the liquid to gas ratio (L/G). Another advantage of the present invention, is that most of the trace contaminants in the landfill gas including chlorinated hydrocarbons, hydrogen sulfide, aromatics, as well as water are removed from the gas stream firstly; by activated carbon adsorption and secondly; during the compression and cooling stages that are condensed and removed in the early stages of compression.
The high percent regeneration of the absorbent liquid flowing from the last flashing vessel is achieved by increasing the temperature and further decreasing the pressure of the absorbent fluid to effect evaporation most of the CO.sub.2 gas which is recycled and combined with the main landfill gas stream flowing to the first stage of the gas compression system. The regenerated carbon dioxide lean absorbent liquid removed from the regenerator vessel is then pumped back to the absorber vessel. The organic absorbent must be nonvolatile, have a low vapor pressure to prevent vapor losses in the regeneration step and must exhibit high selectivity for absorbing carbon dioxide from the methane gas, have low liquid to gas molar ratio (L/G), and have excellent stable physical characteristics. The absorbent capacity to remove CO.sub.2 is normally decreased in the presence of hydrogen sulfide which reacts with the absorbent forming stable sulfides. Filtering of the recycled absorbent fluid and providing make up of the losses by adding fresh absorbent will be required for a continuous operation.
Since organic amine solvents are commercially established and available, the process designer can evaluate and make the selection of absorbent for the process. The physical and chemical composition of the solvent, its absorption and regeneration characteristics are considered outside the scope of this application.